LA GEOMECÁNICA EN LA PERFORACIÓN Y VOLADURA DE ROCAS Presentado en VIII Seminario Internacional de Explosivos - INDUMIL noviembre de 2009, BOGOTÁ Publicado con autorización del autor
ÁLVARO CORREA ARROYAVE
Ingeniero de minas y metalurgia Doctor Ingeniero Mecánica de Rocas Especialista en Técnicas Modernas de Voladura Profesor Escuela de Ingenieros Militares Profesor Universidad Santo Tomás Profesor Pensionado Universidad Nacional de Colombia Gerente Técnico PERVOL S.A.
INTRODUCCIÓN En el diseño de voladuras, las características físicas, químicas y mecánicas de las rocas, (entre otras de sus propiedades) así como la estratigrafía y los rasgos estructurales del macizo rocoso, juegan un papel importante pues determinan determinan la geometría de la voladura, el consumo específico y la regulación de los tiempos de retardo tanto en voladuras a cielo abierto como subterráneas, pero también el consumo de aceros de perforación y las molestias sobre la comunidad y el entorno. Así lo ha dejado muy explícito el Instituto Tecnológico Geominero de España en su excelente obra Manual de Perforación y Voladura de Rocas, en especial en sus capítulos 17 y 18 dedicados al estudio de las rocas y de los macizos rocosos respectivamente. Los elementos básicos de geología que tienen incidencia en la perforación y voladura son: 1) las características físicas y mecánicas de las rocas que conforman el macizo rocoso, 2) la estratigrafía, esto es, la presencia de estratos menos resistentes, y eventualmente más delgados, y presencia de cavidades cavidades y 3) los rasgos estructurales, esto es, la presencia de planos de estratificación, estratificación, diaclasas principales y secundarias, en lo que tiene que ver fundamentalmente con sus actitudes. Otro parámetro que debe tenerse en cuenta en un diseño racional, está obviamente, asociado con los objetivos de la voladura; este otro actor puede conllevar a modificar los diseños en virtud a favorecer la granulometría a lograr, así como a evitar la dilución del material a remover. El principio de una buena voladura se fundamenta en una buena supervisión al proceso de perforación.
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1.- CARACTERÍSTICAS FÍSICO-MECÁNICAS DE LAS ROCAS Los materiales que constituyen los macizos rocosos poseen ciertas características ingenieriles función de su origen y de los procesos geológicos posteriores a los que han podido estar sometidos. Es así como se tienen las rocas ígneas, sedimentarias y metamórficas.
Rocas Ígneas Las masas de roca ígnea homogénea son normalmente densas y la fragmentación uniforme resultante de un procedimiento de voladura y una malla determinada, puede ser menos problemática que la que se tendría en masas rocosas sedimentarias o metamórficas. Los sistemas de diaclasa que existen normalmente, exhibiendo dirección e inclinación con una orientación preferencial específica (diaclasas de enfriamiento) influencian tanto la fragmentación como las condiciones de regularidad del frente y el fracturamiento del macizo rocoso detrás de la última línea volada. En las rocas de origen volcánico, también es frecuente encontrar un gran número de cavidades formadas durante su enfriamiento. Rocas Sedimentarias Las rocas sedimentarias presentan una variación más amplia de problemas de voladura, por ejemplo, las calizas inorgánicas de origen químico son más densas y usualmente algo más frágiles que las orgánicas formadas a partir de conchas y esqueletos de organismos vivos, más porosas y resilientes que las primeras; esto sugiere que para estos casos, puede dar mejor resultado en la voladura un agente de voladura de baja velocidad que dilate la roca, que uno con una velocidad mayor. La consistencia del material cementante en los aglomerados, areniscas y pizarras, influye notoriamente notoriamente en la voladura y fragmentación de estas rocas.
Rocas Metamórficas El bandeamiento en una roca metamórfica tal como el gneiss, tiene una influencia preponderante en la fragmentación. Las voladuras perpendiculares al bandeamiento producirán resultados diferentes a las realizadas paralelas a dicha dirección.
Propiedades físicas y mecánicas de las rocas El análisis que se presenta a continuación es un resumen extraído del manual de perforación y voladuras de rocas del ITGM. Las propiedades físicas y mecánicas más importantes de las rocas que influyen sobre el buen diseño de una voladura, son: • • •
La densidad Las resistencias dinámicas La porosidad
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• • • •
La velocidad de la onda longitudinal El módulo de elasticidad La relación de Poisson El índice de expansión
La densidad. En general, las rocas de baja densidad se deforman y rompen con facilidad, requiriendo un factor de energía relativamente bajo, en tanto que las rocas más densas demandan de una mayor cantidad de energía para lograr una fragmentación satisfactoria así como un buen desplazamiento y esponjamiento del material volado. En rocas con alta densidad, para que el empuje impartido por los gases sea el adecuado, deben tomarse las siguientes precauciones: aumentar el diámetro de perforación para elevar de esta manera la presión de barreno, reducir el esquema y modificar la secuencia de encendido, mejorar la efectividad del retaque con la finalidad de aumentar el tiempo de actuación de los gases y obligarlos a que escapen por el frente libre y no por el mismo barreno y utilizar explosivos con una alta energía de empuje. Este parámetro es quizá el más utilizado en las expresiones que permiten determinar parámetros geométricos tales como el burden, pero también en la determinación de la impedancia.
Las resistencias estáticas. Las resistencias estáticas a la compresión y a la tracción, se utilizan como parámetros indicativos de la aptitud de la roca a la voladura. De esta forma, Hino, 1959, definió el índice de volabilidad, como la relación resistencia a la compresión / resistencia a la tracción, de modo que un mayor valor de esta relación estaría asociado con una mayor facilidad para fragmentar la roca. Conviene igualmente determinar las resistencias a la compresión triaxial estáticas para tener una idea del incremento de esfuerzo de la roca producto de dicho confinamiento. Estos parámetros ingresan en un sinnúmero de formulaciones asociadas con el consumo de aceros de perforación y con el consumo específico, entre otros de los parámetros más importantes de la voladura. En la Figura 1 se muestran algunos de los parámetros mecánicos extraídos de un ensayo de resistencia a la compresión simple sobre un núcleo de caliza.
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800,00 C
,
c
700,00
Fisuración Inestable
e l p m i s
V/V
500,00
F
400,00
C
/2
E50% (E50)
300,00
C
R
200,00
K
s
Cierre de Vacios
Degradación Post-ruptura
Et
Deformación Elástica Lineal
o z r e u f s E
l
R
Fisuración Estable
n ó i s ² e r m c p / m o k c a l a
600,00
Resistencia residual
100,00
El ° 0,00
-4000 t
-3000
-2000
-1000 t°
0 l°
Defo De for. r. Lo Long ng
1000
2000
3000
4000
5000
Deformación 10-6 Defo De for. r. Tra rans. ns.
6000
7000
8000 l
Rela Re lac. c. Volum. u m.
Figura 1. Curvas Esfuerzo de compresión axial – deformación unitaria axial – deformación unitaria diametral – deformación unitaria volumétrica. Donde: Límite de la fase de cierre de fisuras Inicio de la fisuración estable (Griffith) Inicio de la fisuración inestable Resistencia a la compresión simple Resistencia residual E Módulo de deformación tangente Ei Módulo de Young en el origen K Pendiente de la fase lineal de la deformación volumétrica Et Módulo de deformación lateral R Umbral de dilatancia εl Deformación unitaria axial εt Deformación unitaria lateral εlº, εtº Deformaciones residuales ΔV/V Deformación unitaria volumétrica σs σf σl σc σR
Amerita aquí precisar los conceptos de tenacidad, resiliencia y rigidez, los cuales se extraen de una curva esfuerzo-deformación como la indicada en la figura anterior.
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•
Tenacidad
La tenacidad refleja fundamentalmente la capacidad de un material para absorber energía durante la deformación plástica. En una prueba estática, esta energía se mide por el área bajo la curva esfuerzo - deformación, la cual representa el trabajo requerido para fallar la muestra. El módulo de tenacidad, es la máxima cantidad de energía de deformación por unidad de volumen que el material puede absorber sin fracturarse; se expresa en kg.cm/cm³. El módulo de tenacidad, Mt, puede estimarse, para materiales que tienen un comportamiento parabólico en la curva esfuerzo - deformación, como las rocas, mediante la expresión: M t =
2 σ ⋅ ε 3 c f
donde Mt: módulo de tenacidad σc: esfuerzo de la compresión simple deformación unitaria en la falla εf : Puede deducirse entonces, que los materiales con una alta tenacidad deben ser muy resistentes y exhibir una alta ductilidad. Los materiales frágiles, por su parte, usualmente tienen baja tenacidad dado que sólo muestran una pequeña deformación plástica antes de la fractura (Jastrzebski, 1959). Un método simple para medir la tenacidad al impacto de la roca, es el que se realiza en la máquina de impacto Page. La prueba de impacto se realiza sobre núcleos de roca intacta cuidadosamente preparados, de 25 mm de altura por 25 mm de diámetro, relación H/D = 1. Una masa de 2 kg cae verticalmente entre unas guías paralelas, hasta un émbolo terminado en forma esférica que pesa 1kg el cual permanece en contacto con el núcleo. La altura de caída del primer golpe es de 1cm y cada golpe sucesivo posterior, va aumentando 1 cm en altura. La altura en centímetros del golpe que produzca la ruptura del material, se registra como el valor de su tenacidad. El método estándar para determinar la tenacidad de la roca, se presenta con la designación ASTM: D 3-18. Obert et al. (1946) utilizaron este procedimiento, con ligeras modificaciones, para estandarizar las pruebas sobre material rocoso para minería. La tenacidad de la roca también correlaciona con su dureza de rebote. •
Resiliencia
La capacidad de un material para absorber energía en el rango elástico, se conoce como
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límite elástico; se expresa en kg.cm/cm³ y está representado por el área bajo la curva, esfuerzo - deformación, en la zona elástica. Las dos propiedades, esfuerzo en el límite elástico y elasticidad, que al parecer influyen, en gran parte, sobre la resistencia a la compresión, intervienen en la determinación del módulo de resiliencia, tal como se aprecia en la siguiente expresión: 2
M r =
donde Mr : σl : E: εl :
σ
l
2E
módulo de resiliencia esfuerzo en el límite elástico módulo de deformación deformación unitario en el límite elástico
Esta ecuación indica que para un alto módulo de resiliencia, el material debería exhibir un alto esfuerzo en el límite elástico. El término resiliencia no debe confundirse con el módulo de resiliencia. Dentro del límite proporcional, la resiliencia es igual al trabajo externo realizado sobre un material durante la deformación. De este modo, la resiliencia total de un material es el producto de su volumen por el módulo de resiliencia (Eshbach, 1952). Una baja resiliencia es conveniente para una buena amortiguación de la amplitud de las ondas, y una alta resiliencia para la baja generación de calor interno (Richards, 1961). Los valores del módulo de resiliencia en compresión, de algunas rocas típicas ensayadas por la oficina de Minas de los Estados Unidos (Windes, 1949), se indican en la Tabla 1.
Tabla 1. Módulo de resiliencia de algunas rocas típicas Roca Mr (in-lb/in³) Jaspilita 448,0 Hematita 134,0 Anfibolita 124,5 Mármol 66,3 Arena 62,5 Concreto 5,0 El concepto de energía de deformación expresado por el Mr, muestra claramente la influencia de las dos variables, esfuerzo y elasticidad, sobre la falla del material. Aunque el mármol y la arenisca tienen aproximadamente el mismo módulo de resiliencia, es evidente que los minerales que conforman el mármol ofrecen una mayor resistencia a la carga (una deformación más baja) no obstante, la energía total en ambos casos es teóricamente la misma. Cabe anotar, además, que sólo en casos excepcionales (en la determinación estática del módulo de elasticidad del material), existen relaciones lineales entre el esfuerzo y la deformación (Wuerker, 1953).
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•
Rigidez
La rigidez está caracterizada por el módulo de deformación o módulo de elasticidad, el cual es el factor de proporcionalidad entre el esfuerzo normal y la deformación relativa que sufre la muestra de roca; su valor en la mayoría de las rocas varía entre 0,3 x 10 5 y 1,7 x 10 5 MPa, dependiendo fundamentalmente de la composición mineralógica, la porosidad, el tipo de deformación, y la magnitud de la carga aplicada. También influye en dicho parámetro la estructura de la roca, ya que el módulo de deformación en la dirección de la estratificación o la esquistosidad es generalmente mayor que en la dirección perpendicular a ésta. El coeficiente de poisson, por su parte, es el factor de proporcionalidad entre las deformaciones tranversales relativas y las deformaciones longitudinales. Para la mayoría de las rocas y minerales está comprendido entre 0,2 y 0,4 y sólo el cuarzo exhibe un coeficiente de poisson anormalmente bajo, alrededor de 0,07.
Las resistencias dinámicas. El tratamiento racional de los problemas reales obliga a considerar las resistencias dinámicas. Las resistencias de las rocas aumentan con la velocidad de carga pudiendo llegar en algunos casos a alcanzar valores entre 5 y 13 veces superiores a las estáticas. Es por ello que puede ser bastante conveniente, determinar en el laboratorio las resistencias al impacto, (a fin de determinar, por ejemplo, el índice de impacto, relación trabajo específico de impacto a resistencia a la compresión simple) a fin de seleccionar explosivos que desarrollen en las paredes del barreno, esfuerzos inferiores o iguales a la resistencia a la compresión dinámica de la roca o provocar una variación en la curva Presión-Tiempo, mediante el desacoplamiento de la carga dentro del barreno. La comparación entre algunas de las propiedades estáticas y dinámicas de las rocas, se presenta en la Tabla 2.
Tabla 2. Comparación de algunas propiedades dinámicas y estáticas de algunos tipos de rocas, según Coates.
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kg/cm²/seg Esfuerzo de rotura, kg/cm² Deformación en la falla, d, micras E, kg/cm²
53 145 47 x 10 4
80 410 19 x 104
29 370 10 x 104
53 510 12 x 104
De la anterior tabla se desprende que para una velocidad de aplicación de la carga de un millón de veces superior, es decir, ensayos de impacto (o de corta duración), 1.- Se obtiene una resistencia entre 2,75 y 6,55 veces superior a la estática, según el tipo de roca. 2.- El módulo de deformación, aumenta entre 1,08 y 4,00 veces, según el tipo de roca. Lo anterior que indica, entonces que la roca se hace más resistente y más deformable. Las anteriores consideraciones ponen de manifiesto la necesidad de hacer intervenir en los cálculos para voladura el efecto de las propiedades dinámicas en lugar de las estáticas, en diseños de voladura. Según esto, entonces, la recomendación es, en principio, tomar un valor de por lo menos 5 veces la resistencia a la compresión simple como su equivalente en resistencia a la compresión simple dinámica, en tanto que su módulo de deformación, pudiera ser del orden de 2 veces el correspondiente al módulo en compresión simple. Con estas consideraciones, puede entrarse ahora a deducir la tenacidad y la resiliencia dinámicas, fundamentales para el cálculo energético de las voladuras.
La porosidad. Se trata de la porosidad intergranular, primaria o de formación, cuya distribución en el macizo pudiera considerarse uniforme y que genera efectos como la atenuación de la energía de la onda de choque y la reducción de la resistencia resistencia dinámica a la compresión y, consecuentemente, un incremento de la trituración y porcentaje de finos. El trabajo de fragmentación en rocas muy porosas, se realiza, casi en su totalidad, por la energía de empuje, lo que obliga a utilizar explosivos con una elevada energía de empuje, sacrificando la energía de tensión, mediante el desacoplamiento de las cargas y a retener los gases de las voladuras a alta presión con un adecuado dimensionamiento de la longitud y tipo de retaque.
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La conductividad. Fugas o derivaciones de corriente pueden presentarse cuando los detonadores se colocan dentro de barrenos en rocas de cierta conductividad, como los sulfuros complejos o las magnetitas, y esencialmente cuando las rocas son abrasivas y existe agua en el entorno de la pega. La velocidad de la onda longitudinal . La velocidad de la onda longitudinal es la velocidad a la cual la roca transmitirá las ondas de compresión: mientras más densa y homogénea sea la roca, más eficientemente se propagarán las ondas. La velocidad de detonación del explosivo requerido, se determina mediante las características de propagación de las ondas en la roca, estando en una relación directa. Este parámetro es vital en el buen manejo de una voladura, pues determina no sólo la impedancia del terreno sino también, es un parámetro que permite determinar la vibración que producirá el explosivo en el terreno.
El módulo de elasticidad . Es la medida de la rigidez de la roca; esto es, la medida de la capacidad que tiene la roca a contrarrestar o resistir la deformación. Mientras mayor sea este módulo, mayor oposición presentará la roca para ser fracturada. La relación de Poisson. Mide el grado en el que un material dilata o comprime su área de sección transversal perpendicular al esfuerzo. La relación de Poisson no tiene ninguna influencia en la determinación de las características de la voladura de las rocas excepto que una relación menor está asociada con la mayor propensión de la roca al prefracturaminento (presplitting). El índice de expansión. Se refiere al aumento de volumen que ocuparía un metro cúbico de roca en banco, al ser volado. Depende de la granulometría y de su grado de acomodación. Las anteriores, y otras propiedades de las rocas influencian tanto la perforación como la voladura, lo cual es bien conocido por los expertos en explosivos, mas lo que se percibe bastante claramente es que aun así se ignoran, lo cual puede dar lugar a voladuras menos eficientes.
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La palabra dureza como se emplea e mplea en la industria de la perforación, tiene una variedad de significados que dependen incluso del método de perforación utilizado. El término, “roca dura”, se usa generalmente para describir una formación geológica que por alguna razón es difícil de perforar. En la perforación con broca de diamantes, la dureza se interpreta como la resistencia a la abrasión. En la perforación por percusión, el término implica la resistencia al impacto, mientras que con las perforadoras de rotación, la dureza se considera análoga al esfuerzo de compresión. Aunque éstas se consideren variables enfrentadas para los diferentes tipos de roca, el término dureza se aplica indiscriminadamente por la industria, como la resistencia de la roca a la penetración por cualquier tipo de técnica de perforación (Mather, 1951). El volumen de material perforado con taladros neumáticos, en la industria minera, petrolera y de la construcción, ha dado excelentes resultados. Por tanto, el problema del aumento en la eficiencia de la perforación toma una gran importancia; éste depende de las propiedades mecánicas del material rocoso, las herramientas de perforación y el operario encargado (Protodyakonov, 1963). Numerosas investigaciones han sido llevadas a cabo por los fabricantes de equipos de perforación, acero para taladros, brocas y otros componentes, con el fin de determinar los tipos y especificaciones más adecuadas para perforar diferentes estratos. La mayoría de los investigadores aceptan que, aunque un conocimiento de las propiedades físicas es útil, en general, éstas no son una guía segura para definir la perforabilidad. Rollow (1963) describe una prueba de laboratorio en la cual, la perforabilidad puede predecirse sobre la base de perforar pequeñas muestras con una micro broca de un diámetro de 1¼”. Al perforar las muestras de la formación rocosa con esta microbroca y al medir la tasa de perforación y el desgaste de los dientes, pueden establecerse estimaciones para el desempeño de una broca de mayores dimensiones. El primero en reportar el uso de la microbroca fue Scott (1946) en la que correlaciona la perforabilidad de la roca con el esfuerzo a la trituración. Head (1951) utilizó una microbroca para establecer una clasificación de perforabilidad para 15 formaciones geológicas diferentes. Mather (1951) indica que la causa principal de confusión, en la literatura de perforación de rocas, puede atribuirse a las diferentes definiciones del concepto de dureza de las rocas y
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con un ensayo muy elaborado, de perforación. Head (1951) realizó mediciones usando la prueba Knoop, con el fin de determinar la relación entre la perforabilidad de siete diferentes formaciones geológicas y la dureza de las muestras de esas formaciones, y concluyó que no existe una relación consistente entre la dureza y la perforabilidad de estas formaciones; mas sin embargo, concluyó en que la perforabilidad se relaciona más con la forma en que los cristales duros se encuentran cementados. Las pruebas de dureza de rebote también han sido utilizadas para determinar sus aplicaciones como un indicador de perforabilidad. Wolansky (1949) hizo uso del esclerómetro en Alemania para obtener valores seguros y comparables de dureza de las rocas, en particular, con respecto a la perforabilidad. Los ensayos concluyen que el esclerómetro puede ser útil para indicar la perforabilidad, si se tienen en cuenta los valores comparativos, antes que los números absolutos.
1. Dureza Total Deere (1970) sugirió el uso de una combinación entre resistencia y abrasividad de la roca para la previsión de la penetración de tuneladoras y llamó Dureza Total (HT) al parámetro H T = H R H A
donde HT : dureza total HR : dureza de rebote al martillo Schmidt HA : abrasividad Taber Modificado Nelson et al. (1983) han estudiado cuatro túneles y encontraron una excelente correlación entre el índice de penetración y la dureza total I d = 5,95 + 0,18H T
donde Id : Índice de penetración, kN/mm HT : dureza total
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2.- ESTRATIGRAFÍA Las voladuras en zonas en donde se produce un cambio litológico drástico y, consecuentemente, una variación de las propiedades de las rocas, obligan a una reconsideración de los diseños. diseños. Deben colocarse, colocarse, por ejemplo, espaciadores en arcilla o gravilla entre la carga del barreno en donde la presencia de lodolitas, por ejemplo, indica que es posible una explosión prematura. A menudo, estratos de lutita o arcillolita muy blandos, se encuentran concentrados en una pequeña longitud del barreno, los cuales deben ser tratados de la misma manera a fin de prevenir una pérdida de energía del explosivo. Los yacimientos estratiformes semi horizontales que presentan algún horizonte muy resistente, pueden conducir a un tipo de voladuras particular en los que las cargas se alojen perfectamente confinadas, a la altura de tales horizontes. Igualmente, es recomendable que la localización de los multiplicadores en las columnas de explosivo coincida con los niveles más resistentes a fin de aprovechar al máximo la energía de tensión producida por dicho explosivo. Rocas y sus intervalos de dureza conocidos
ROCAS Y SUS DUREZAS MEDIAS 200
Rocas Ígneas
Cuarcita
a t i c r a u C
190 180 170
Rocas Metamórficas
s a
a t i c r a u
Rocas Sedimentarias
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Cuando se encuentren en contacto dos materiales de resistencias muy diferentes, como por ejemplo una lutita físil en contacto con una arenisca muy competente, y si los barrenos atraviesan dichas formaciones, tendrá lugar una gran pérdida de energía asociada con la caída de presión y escape de los gases al producirse deformaciones rápidas de dichos materiales menos resistentes y, por consiguiente, se obtendrá una mala fragmentación (esta situación, que de hecho es mucho más común de lo que se pueda pensar, es un buen ejemplo para llamar la atención sobre el seguimiento que debe hacerse durante la perforación). Para aumentar el rendimiento de las voladuras en estos casos, se aconseja: retacar con material adecuado aquellas zonas del barreno que estén en contacto con el material menos resistente o próximo a ellas, emplear cargas de explosivo totalmente acopladas a la roca competente con una gran velocidad de detonación y una relación “ET / EB” alta, colocar los multiplicadores en el punto medio de la roca dura para incrementar la resultante de la onda de tensión que actúa a ambos lados y evitar el escape prematuro de los gases a la atmósfera asegurando que tanto la longitud de retaque como la dimensión del burden sean las correctas. Si los materiales de los estratos son diferentes en sus respuestas a la voladura, el piso del banco puede llegar a ser muy irregular; igualmente, es probable que se presenten fracturas terminales si el disparo se inicia en la dirección equivocada. En las Figuras 3, 4, 5 y 6, se ilustran algunas de estas situaciones en macizos rocosos heterogéneos.
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Figura 4. Perfil de terreno mostrando una estratigrafía bastante inclinada y diferentes espesores de banco. Supóngase que el objetivo sea beneficiar el banco oscuro. En estas figuras, los entramados hacen alusión a las diferentes características tanto físicas como mecánicas, tales como: localización del estrato con respecto a la superficie, espesor del mismo, peso unitario, estado de diaclasamiento, resistencia a la compresión simple y módulo de Young, entre otros parámetros, así como los objetivos de la voladura.
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e ncuentra inmediatamente Figura 6. En este caso, el estrato a beneficiar sería el que se encuentra encima del estrato en negro.
3.- RASGOS ESTRUCTURALES
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Cuando la estratificación o los sistemas de diaclasas presentan un ángulo menor de 30º, es recomendable que los barrenos sean normales a dichos planos con la finalidad de aumentar el rendimiento de las voladuras. Si se conoce la distribución en planta del estado de diaclasamiento del macizo, debe tenerse en cuenta esta disposición a fin de colocar correctamente los explosivos con los espaciamientos apropiados, en lugar de mantener un espaciamiento constante. En efecto, situaciones en las que las diaclasas se presentan en familias formando ángulos suplementarios, originan liberaciones desiguales de la energía de los explosivos, llegando la roca a fragmentarse excesivamente en las zonas con ángulos agudos y produciendo grandes bloques en las zonas con ángulos obtusos. Para evitar estos inconvenientes, que influyen muchas veces en la transmisión de importantes vibraciones al terreno, las cargas de explosivo deben colocarse preferentemente junto a zonas con ángulos obtusos y el espaciamiento entre barrenos debe ser paralelo a las direcciones de los planos de fractura. Además de estos procedimientos, se recomienda una reprogramación de las secuencias de encendido de las cargas, con la finalidad de crear la máxima superficie libre después de cada detonación, circunstancia que depende de la geometría de fracturación del macizo rocoso. Cuando es posible modificar el diámetro de los barrenos, conviene utilizar diámetros más pequeños al interior de zonas más fracturadas, pa ra controlar mejor la fragmentación y los impactos ambientales resultantes. Las discontinuidades en el macizo rocoso tienen gran influencia sobre el diseño y
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La orientación de las discontinuidades respecto al frente de explotación, también tiene influencia en la eficiencia de la voladura, tal como se puede apreciar en las Figuras 9 y 10.
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Otras alternativas para implementar el control geoestructural, consisten en dotar a las voladuras de líneas de barrenos de precorte, las cuales reducen la probabilidad de sobrefracturación del macizo remanente, aunque pueden ocasionar vibraciones excesivas. Las tendencias son, por tanto, utilizar patrones de voladura versátiles, que se adapten a las discontinuidades de los macizos, lo que demanda un previo conocimiento de éstas. En una formación con presencia de estratos inclinados, los barrenos deben perforarse en filas a fin de crear una cara orientada en la dirección de buzamiento de dichos estratos (es decir, normal a la dirección de los mismos). Este procedimiento incrementa igualmente la posibilidad de tener una roca fracturada más allá de los límites de la última fila de barrenos, como en todos los casos, pero la parte superior posterior del último plano de estratificación se proyecta hacia afuera y la masa rocosa remanente no soportada es
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intercomunicarse el barreno con las cavidades, y, como consecuencia, la interrupción de la apertura de grietas radiales al escapar los gases hacia los espacios vacíos. Otra de las características del macizo rocoso que influencian la voladura son, los esfuerzos in-situ; en este caso, el patrón de fracturas generado entorno a los barrenos, puede estar influenciado por la concentración no uniforme de esfuerzos alrededor del mismo. En rocas masivas homogéneas, las grietas que empiezan a propagarse radialmente desde los barrenos, tienden a seguir la dirección de los esfuerzos principales. Esto quiere decir, entonces, que después de determinar la dirección de los esfuerzos principales, el frente de voladura debe orientarse en esa misma dirección. La presencia de agua, también tiene una importante influencia en el buen desempeño de las voladuras. Las rocas porosas y los macizos intensamente fracturados, cuando se encuentran saturados, presentan habitualmente ciertos problemas que obligan a seleccionar explosivos que no se alteren por el agua; además dicha humedad incide
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El tamaño de bloques es uno de los indicadores más importantes del comportamiento del macizo rocoso. Sus dimensiones se determinan mediante el espaciamiento, el número de familias y la persistencia de las discontinuidades que delimitan los posibles bloques. El número de familias y la orientación determinan la forma de los bloques resultantes, los cuales pueden asemejarse a cubos, romboedros, tetraedros, láminas etc. De cualquier modo, las formas geométricas regulares son la excepción a la regla ya que rara vez las discontinuidades resultan paralelas. Usualmente las rocas de tipo sedimentario presentan los bloques más regulares. La eficiencia de la voladura y los procesos de extracción de materiales de cantera, están fuertemente afectados por el tamaño natural del bloque in situ. El tamaño de bloques puede describirse mediante la dimensión media del bloque típico (índice modal de tamaño de bloques, Ib) o mediante el número total de discontinuidades que intersectan un volumen unitario del macizo (conteo volumétrico volumétrico de diaclasas o número de diaclasas por
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del espaciamiento. Todas las distancias, d, entre discontinuidades adyacentes, se miden y registran sobre longitudes de muestreo muestreo no inferiores a 3m (o al espesor de la unidad de roca que se observe, si ésta es menor de 3m). La longitud de muestreo debería ser
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Conteo volumétrico de diaclasas o número de diaclasas por metro cúbico, Jv . Es la sumatoria del número de diaclasas por metro para cada familia de diaclasas. Las discontinuidades casuales pueden incluirse pero generalmente tienen muy poco efecto
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Bloques de tamaño medio
12 a 20
50 a 75
Regular
Bloques pequeños
20 a 27
25 a 50
Mala
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Usualmente las voladuras en macizos rocosos tienden a abrir las fracturas preexistentes; por tanto, el tamaño de los bloques medidos a partir de la roca arrancada y el depósito de
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